When delivering drugs through the skin (transdermal), various methods and devices have been proposed to enhance permeability of the skin. In one method, microneedles or blades having certain lengths are used to pierce the stratum corneum without passing completely through the epidermis.
In another method for transdermal delivery of drugs, microabraders are used to abrade the stratum corneum of the skin, the microabraders having an array of microneedles with a frustoconical shape, each having a length of about 50 to 250 microns. They also are used to rub against the surface of the skin.
Transdermal drug delivery is also known to use pulsed laser light to ablate the stratum corneum without significant ablation or damage to the underlying epidermis. A drug is then applied to the ablated area and allowed to diffuse through the epidermis.
Other methods of increasing skin permeability include chemical permeation enhances, electrical methods (electroporation), ultrasonic means (sonophoresis), and an electrical field across skin (iontophoresis).
In transdermal drug delivery using microabraders, the required dimensions of microneedles on a microabrader pose significant manufacturing challenges. When machining microabraders, the thin walls of the microneedles are subjected to stresses caused by the cutting tool. This may result in structural failure or damage to the microneedle during a machining operation. Such structural failure or damage is more evident when machining microneedles with higher aspect ratios.
Other methods of manufacturing microneedles include photolithographic etching, which is slow, and does not produce a mirror surface finish.
Furthermore, prior processes result in an unsatisfactory surface finish; an uncontrolled profile of the microabraders; excessively long machining times; and a restricted range of materials are able to be used—mainly silicon. For example, etching requires about 1 minute/μm. For a microabrader 250 μm high, the etching time is 250 minutes.